P0128 on a 2020 Jeep Wrangler Unlimited

P0128 means the engine warmed up slower than expected or stayed cooler than the computer expected.

Not your vehicle? See general P0128 info.

Most common causes on the 2020 Jeep Wrangler Unlimited

Ranked from 597 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant loss from the recalled water-outlet housing / coolant reservoir (TSB/recall ZD8)
High severity
Multiple complaints on this VIN describe the water-outlet housing bolts failing or the coolant reservoir cracking and going on backorder, repeatedly draining the cooling system.
Reported in 6 NHTSA complaints
Parts:coolant reservoir water outlet housing
2
Cooling fan stuck running continuously
Medium severity
One owner reported the engine fan staying on for years, with the dealer dismissing it as needing to 'cool off' — a fan that won't cycle off over-cools the engine below its regulating temperature.
Reported in 1 NHTSA complaint
Parts:cooling fan relay cooling fan
3
Leaking or faulty thermostat housing
Medium severity
One owner found the thermostat housing leaking during a routine inspection; a leak here often accompanies a thermostat that fails to seat and close properly.
Reported in 1 NHTSA complaint
Parts:thermostat thermostat housing
4
Stuck-open thermostat
Medium severity
A thermostat that won't close lets coolant circulate continuously through the radiator, keeping the engine below its regulating temperature.
Common cause
Parts:thermostat
5
Failed or inaccurate ECT sensor
Medium severity
A drifting or open-circuit coolant temperature sensor reports a false low reading even at full operating temperature.
Common cause
Parts:coolant temperature sensor
6
Air pocket in the cooling system after a coolant top-off or repair
Low severity
Trapped air prevents the sensor from reading true coolant temperature until the system is fully bled.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0128 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Weak cabin heat
Poor fuel economy
Temperature gauge low
Long warm-up time

What to check before replacing parts

Check coolant level when the engine is cold

Look for: Low coolant reduces the system's ability to reach and hold operating temperature — top off and recheck before assuming the thermostat or sensor is at fault.

Compare scan-tool coolant temperature to actual warm-up time

Look for: An engine that takes noticeably longer than normal to reach operating temperature (or never gets there) on the scan tool, with normal coolant level, points to the thermostat rather than the sensor.

Inspect thermostat operation

Look for: A thermostat stuck open (or opening at the wrong temperature) lets coolant circulate through the radiator too early, keeping the engine running cooler than it should — this is the most common cause of this code.

Check cooling fan command

Look for: A fan that runs constantly or turns on too early (commanded on when temperature is still low) keeps the engine from warming up normally, mimicking a stuck-open thermostat.

Inspect coolant temperature sensor data

Look for: A sensor reading that doesn't rise smoothly with actual engine warm-up, or that reads implausibly low compared to how the engine feels/behaves, points to a sensor or wiring fault rather than a true cooling problem.

Rather have a pro handle it? Find a vetted local mechanic.